Fused electrical safety barrier for protection of an electrical load placed in a potentially hazardous location

Abstract

To protect an encapsulated fuse (9) within a wholly enclosed sealed safety barrier having a first voltage limiting circuit (10) and a current limiting circuit (15), an externally accessible high current interrupter (5), such as a fuse, positive temperature coefficient resistor (26) or circuit breaker (27) is connected in series with an input terminal, and further serially connected with a second voltage limiting circuit (6) connected downstream of the interrupter and across the safety barrier circuit, the second voltage limiting circuit having a voltage limiting response characteristic which is, preferably, the same as the first voltage limiting circuit within the barrier, and the interrupter having a response threshold which is at least as high, but may be higher than the fuse (9) encapsulated within the circuit. The combination of the second voltage limiting circuit (6) with the fuse (5) acts as a clamping circuit, in case of over-voltage or other malfunction, with respect to the encapsulated fuse (9) so that the externally accessible interrupting element can be replaced or reset, or will reset automatically to avoid replacement of the entire safety barrier.

We claim: 
     
       1. Safety barrier having an input (2) including two input terminals (3, 4) and an output (21) including two output terminals (22, 23), and a barrier circuit (10, 15) limiting the voltage across the output and the current flowing through the output,   wherein at least part of said barrier circuit forms a sealed, encapsulated unit (24);   said barrier circuit comprising   a first voltage limiting circuit (12);   a first current limiting circuit (15) serially connected between the voltage limiting circuit and at least one (22) of the output terminals; and   an in-line fuse (9) located within said sealed unit (24) and inaccessible from the outside thereof, serially connected between one (3) of the input terminals, and the voltage limiting circuit (12),   wherein means are provided for protecting said encapsulated unit against overvoltages and surges, comprising   a series circuit including   an externally accessible high current interrupting means (5, 26, 27) connected to one of the input terminals and   a second voltage limiting circuit (6);   said first voltage limiting circuit (10) having an input which is connected to said second voltage limiting circuit (6);   said high current interrupting means (5, 26, 27) being connected between said second voltage limiting circuit (6) and one (3) of the input terminals and thereby connecting said first voltage limiting circuit through said high current interrupting means (5, 26, 27) to said one (3) of the input terminals; wherein the current interruption threshold of the externally accessible high current interrupting means (5, 26, 27) is at least as high as the interruption threshold of said in-line encapsulated fuse (9); and   wherein said high current interrupting means (5, 26, 27) is accessible externally of said sealed, encapsulated unit (24).   
     
     
       2. The barrier of claim 1, wherein the voltage threshold limits of said first voltage limiting circuit (10) and of said second voltage limiting circuit (6), at least at a given polarity of voltages connected to the input (2),are the same, subject to tolerances, for limiting voltage across said circuits to said threshold values. 
     
     
       3. The barrier of claim 1, wherein the second voltage limiting circuit (6) and the first voltage limiting circuit (10) both have limiting voltage threshold which, at a given polarity of voltage connected to the input (2), is higher than when said polarity is reversed. 
     
     
       4. The barrier of claim 1, wherein the voltage thresholds of the second voltage limiting circuit (6) and of the first voltage limiting circuit (10) are identical, subject to tolerances, and independent of the polarity of the voltage across the input (2). 
     
     
       5. The barrier of claim 1, wherein the externally accessible high current interrupting means comprises a fuse (5). 
     
     
       6. The barrier of claim 5, wherein both the in-line fuse (9) and said externally accessible fuse (5) are melt-through fuses. 
     
     
       7. The barrier of claim 1, wherein the externally accessible high-current interrupting means comprises a positive temperature coefficient resistor, or cold conductor. 
     
     
       8. The barrier of claim 1, wherein the externally accessible high current interrupting means comprises a circuit breaker (27). 
     
     
       9. The barrier of claim 1, wherein the current response time of the externally accessible high current interrupting means (5, 26, 27) is longer than the current response time of said in-line fuse (9). 
     
     
       10. The barrier of claim 1, wherein the second voltage limiting circuit (6) has a limiting threshold level which is smaller than the maximum voltage at which said in-line fuse (9) responds and burns out. 
     
     
       11. The barrier of claim 1, wherein at least one of: the first voltage limiting circuit (10); the second voltage limiting circuit (6); include at least one Zener diode (6, 6'; 12, 12'). 
     
     
       12. The barrier of claim 1, wherein at least one of: the first voltage limiting circuit (10); the second voltage limiting circuit (6); include at least a diode (6", 12"). 
     
     
       13. The barrier of claim 1, wherein said second voltage limiting circuit (6) is located externally of said sealed, encapsulated unit (24). 
     
     
       14. The barrier of claim 1, wherein the current response characteristics of said externally accessible current interrupting means (5, 26, 27) and of said in-line fuse (9) are, subject to tolerances, identical. 
     
     
       15. The barrier of claim 1, wherein the current response characteristics of said externally accessible high current interrupting means (5, 26, 27) is higher than the current level at which the in-line fuse (9) responds.